The invention relates generally to an ellipsometric method for studying physical properties of a sample or testpiece, and an apparatus for carrying out the method. The physical properties of the testpiece, which are the subject of study in accordance with the method and the apparatus, include the properties of a surface of the testpiece or one or more surface layers or films on the testpiece.
The term ellipsometry is used as a collective term for denoting various methods for studying the physical properties of testpieces by means of their properties of changing the polarisation of a polarised light beam. In principle, a beam with a known state of polarisation is reflected at or transmitted through the testpiece. After reflection at the testpiece or after the light has passed through the testpiece, the physical properties of the testpiece can be ascertained from the change in polarisation, using optical calculations.
In reflection ellipsometry, the testpiece has a reflective surface on which the light beam impinges, with an oblique angle of incidence. The ellipsometric data obtained can be used for ascertaining the refractive index and the extinction coefficient of the surface material of the testpiece. If the testpiece is covered with a dielectric film, it is possible to determine the thickness and the refractive index of the film. Reflection ellipsometry is the most sensitive and most accurate method for measuring such films or layers.
Instead of using a reflected beam, it is also possible to analyse the light beam which is transmitted through the testpiece. That method is referred to as transmission ellipsometry.
The same procedure which is used for measuring the change in polarisation in transmission ellipsometry can also be employed for studying bulk properties of transparent materials, for example the birefringence of crystals or optical rotation of a sugar solution. The latter is conventionally referred to as polarimetry, but it falls within the general term of ellipsometry.
For the sake of simplicity hereinafter, reference will primarily be made to reflection ellipsometry but it should be appreciated that, unless stated otherwise, the information and procedures set forth also apply to transmission ellipsometry and polarimetry as referred to above.
In ellipsometry, the physical parameters of the testpiece, which are the aspects of interest, simultaneously affect the relative intensity and the phase delay of the two polarisation components of the light beam. As external influences affect the two polarisation components to the same degree, ellipsometry is very insensitive with respect to such external influences. That also explains the extremely high degree of accuracy of ellipsometric measuring methods in conventional laboratory equipment.
Ellipsometry is a highly developed art, and there are many publications relating thereto, for example R. N. A. Azzam and N. M. Bashara `Ellipsometry and Polarised Light`, North Holland Publishing Co, New York, 1977, disclosing various aspects of ellipsometry. In regard to ellipsometric methods, a distinction is made in particular in regard to the instrument used (also referred to as the ellipsometer) and the procedures used for extracting and interpreting the measurement data.
An ellipsometer essentially comprises a light source for emitting polarised light, a testpiece and an analyser which analyses polarisation of the light after it has been reflected at the testpiece or after it has been transmitted through the testpiece. More particularly, an ellipsometer comprises a light source and a detector together with two polarisers, one of which is disposed near the light source and is conventionally referred to as the polariser, while the other is disposed in the vicinity of the analysing means and is generally referred to as the analyser. The testpiece is disposed between the polariser and the analyser, and the assembly may include one or two devices for altering or modifying the polarisation of the light, referred to as polarisation modifying devices or polarisation modulating devices. The polarisation modulating devices may be polarisers which produce a partial polarisation effect, or birefringent devices (which are referred to as compensators), or optical rotators and/or geometrical rotators. If for example the polariser is rotated relative to the testpiece, that is a geometrical rotation. The differences between the individual kinds of ellipsometers arise out of the choice of the device used for modifying the state of polarisation. The basic construction of an ellipsometer with a compensator (.lambda./4 plate) as the means for modifying the state of polarisation of the radiation is described for example in Journal of Physics E; Scientific instruments, volume 6, No. 5, May 1973, by W. E. J. Neal et al; `Ellipsometry and its applications to surface examination`, pages 409 to 413, in particular page 410.
By virtue of the particular design construction selected, ellipsometers have different properties, for example in regard to accuracy, high measuring speed and suitability for operation with multiple wavelengths.
Because of the non-directional nature of optical laws, the sequence in which the optical components are disposed may be interchanged, between the two polarisers. The mode of operation of the overall assembly then remains the same although the actual condition of the optical components must be taken into consideration when analysing the measurement data.
Ellipsometric methods can be essentially divided into photometric ellipsometry and null ellipsometry. In null ellipsometry, the change in the state of polarisation which is caused by the testpiece is compensated by suitable adjustment of the polarisation modulating device so that the light beam is extinguished by the analyser. Adjustment to a minimum level of received intensity may be effected either manually or automatically. The measurement result is then the position of the polarisation modulating device, upon extinction of the light beam. Such a method is disclosed for example in published European patent application No. 80 101993.6 (publication No. 0 019 088).
In photometric ellipsometry, the devices for altering the state of polarisation are varied in a predetermined manner and the intensity of light reaching the detector is measured for each setting of the polarisation modulating device. The ellipsometric data for the testpiece are then calculated using mathematical models for the respective instrument.
Adjustment or setting of the polarisation modulating device may be effected by rotatable modulator members, wherein one or both polarisation modulating devices is or are continuously changed by rotation of the optical components thereof, which are of a rotationally asymmetrical construction, thereby continuously changing the state of polarisation of the light beam. In that connection, the rotary movement of the polariser or a compensator is frequently effected at a constant speed about an axis of rotation which is parallel to the path of the light beam and the waveform of the received signal is measured during that procedure.
Another photometric ellipsometric method provides using one or more electro-optical polarisation modulating devices for varying the state of polarisation, the modulation properties of such devices being suitably controlled for that purpose and the waveform thus being measured.
Also known are ellipsometric beam division methods wherein the beam, after being reflected at the testpiece or after passing through the testpiece, is split into two or more light beams, with the split beams being measured by different detectors. Different polarisation modulating devices are provided for the split beams. The properties of the testpiece in question can be ascertained on the basis of knowledge of the properties of the polarisation modulating devices and the measured intensities.
The null ellipsometric method requires a detector which is sensitive to the radiation, but not a detector which provides for quantitative measurement, in other words, the naked eye of the operator is sufficient. Although a relatively high degree of accuracy is achieved in that context, the mode of operation is slow. The degree of accuracy depends on the accuracy with which the settings of the polarisation modulating device can be read off. In many cases, this involves optical elements which are rotated by mechanical means. Photometric ellipsometric methods depend for their accuracy on the measuring accuracy of the detector which receives the light intensity. In the case of components of the ellipsometer which are adjustable by a rotary movement, it is necessary to ascertain and measure the angular position of those components as well as the received light intensity, to a very high degree of accuracy. As the light intensities are measured at different times, fluctuations in the light source will have a detrimental effect on the measurement result. In the case of the beam division method, although variations in the intensity of the light source do not have a disadvantageous effect, changes in sensitivity between the individual detectors and receivers will adversely affect the measurement result.